I don't know much about the .Net version, hope it goes well with him but it's not maintained by us.
Still I think you have luck, as a similar limitation applies to the Java version so I might be able to give some hints:
for each type a DocumentBuilder is created at startup, a DocumentBuilder is the object used to transform a specific entity into it's Lucene Document counterpart.
This DocumentBuilder is created at startup and is linked to the Record type, but only knows about the static types you declared, so it doesn't know about the CaseYear and CaseSequenceNumber in the scope of creating a Lucene document for a Record.
you'll have to change the mapping to work around this, or use a custom ClassBridge for Record. A classbridge is able to add all fields it wants dynamically.
It's quite a limitation, please comment here as well as you have thoughts on it: HSEARCH-438
Even if we fix it in the Java version, I have no clue how long it will take, and if anybody will do at all, to bring it into the .Net worlds.